Why did I do it? I still don't really know -- probably because it was the sort of challenge I could set myself with a reasonable chance of not flunking it half way through. It wasn't climbing Mount Everest, or hacking my way through the jungle to find the source of the Amazon, but for a sixty-something anti-athlete, it was, in its own little way, an achievement.
It's 184 miles from the source of the River Thames in a remote Cotswolds meadow to those vast stainless steel shell-like structures of the Woolwich Barrier at the head of the estuary. I've walked every step of the way, 15 days at an average of 12 miles a day, and all of it, as the guide book so encouragingly points out, gently downhill.
So, reason number one to walk along the Thames from start to finish: no steep hills, and no dangerous climbs. Reason number two: you will pass through some of the most glorious countryside that England has to offer: open meadows, ancient woodlands, historic towns and villages.
It was gently drizzling as I set off to find the source in late May -- striding off the train at Kemble station, not far from Swindon, and soon up to my ankles in mud as I followed the signs. I felt grimly determined: whatever the weather, I would not be deterred. Little did I know that I was about to hit the longest heat-wave we've had for seven years.
It's best to be honest: there's nothing to see at the source of the Thames. No burbling spring, no babbling brook. Just a lump of stone saying, in effect: "Guess what, this is the source of the River Thames." And a sign, helpfully pointing vaguely across the field: London Barrier 184 miles.
They say the Thames is like a silver ribbon, threading its way through middle England and much of England's history. Take Shifford, in Oxfordshire, for example, where legend has it that King Alfred summoned a sort of proto-parliament in 890 AD (legend may not be wholly accurate, it has to be said). And of course there's Windsor Castle, Runnymede, the great palace of Hampton Court, Westminster, the Tower of London -- I've walked past them all and marvelled at the richness of what the Thames has seen over the centuries.
I've also stood in flower-rich meadows, mesmerised by thousands of electric-blue dragon flies; I've tried in vain to identify the songs of countless birds; and I've given a wide berth to herds of cows munching their way to happiness. For a townie like me, who's spent much of his professional life obsessed with the big global picture, it was good for the soul to be re-acquainted with the things that make life worth living.
I stumbled, by chance, across the first day of this year's Henley Royal Regatta -- I've never seen so many striped blazers in my life -- and I've gawped, open-mouthed, at some of the opulent river-side residences, with their perfectly manicured lawns and gleaming motor launches.
I've discovered, surprise, surprise, that walking is the speed at which we were designed to move. It's slow enough to be able to take in all the joys of the journey, and there's none of that messing about waiting at locks that you get when you potter about in boats. You can run or cycle if you insist, but you'll miss the delights along the way. I'll stroll, thank you, in tune with the stillness and tranquillity of a river that knows exactly where it's going, and when it'll get there.
So many places stick in the mind: Lechlade, the furthest upstream it's possible to go by boat, with its Halfpenny Bridge and lovely market square; Marlow, with its elegant suspension bridge, used as a model for the one across the Danube linking Buda and Pest in Hungary; and Boulter's Lock in Maidenhead, the place-to-be-seen for Victorian high society.
But when I eventually reached London, I had a decision to make: north bank or south bank? I'm a north Londoner born and bred, so, always the adventurer, I chose south, and found myself skirting endless construction sites in Bermondsey, Rotherhithe and Deptford. And then, on the hottest day of the year so far, I slogged my way round the O2 and the Greenwich Peninsula, past the cement works, and on to journey's end, the Barrier at Woolwich, the squawking of the gulls and the smell of the sea. I bade the river a fond farewell, and headed home -- by boat.